1. Field of the Invention
This invention relates to the alignment and control of segmented aperture optical systems, and more particularly to methods for estimating errors associated with piston misalignments between the various elements of the segmented optical surfaces.
2. Description of the Related Art
A wide variety of proposed large aperture, imaging optical systems presently under discussion may utilize segmented optics technology in order to reduce optics fabrication costs. Such systems will require active sensing and control of segment alignment errors if they are to achieve diffraction-limited levels of image quality. Although a variety of metrology concepts are under development and appear well suited for the direct measurement of segment tilt and piston errors, alignment estimation concepts based directly upon optical system image quality remain of interest to reduce the calibration accuracy and long-term stability required of local sensing techniques. This invention is a new scene-based alignment sensing technique with several significant advantages over existing approaches.
To successfully provide high resolution imagery from distant extended scenes, relatively large aperture optical systems are required. The aperture diameters may be on the order of several meters. In order to reduce fabrication costs, as well as provide a means to package large optical systems into current space launch vehicles, the optical elements, such as mirrors, are fabricated as a number of segmented components. If the segments are perfectly aligned with each other the total prescribed overall mirror surface is formed.
Ideally, all of the mirror segments should lie in a surface that satisfies the mirror prescription. Each mirror segment is provided with actuators that are capable of controlling the piston degrees of freedom by moving the segment forward or backward to align it with adjacent segments. Misalignments in which one segment is either too far forward or too far back with respect to the other segments are referred to as "piston errors".
Large optical systems employing segmented optics require active sensing and control of piston errors to achieve diffraction limited image quality. While various approaches have been taken in the past to detect and correct piston errors, many do not involve direct full segment measurement of the optical system's performance, require optical element "dithering", or are highly sensitive to noise.
The basic problem in determining segment to segment piston errors while observing incoherent extended scenes is the necessity to develop a measurement and processing procedure that separates the scene effects from the optical system's wavefront errors. One prior approach involves the active measurement of piston errors between individual segments by electro-mechanical measurement of the back surface distance variation between segments. This approach suffers from being only an indirect measurement of segment errors. Since the entire front surfaces of the segments (not the rear surfaces) provide the critical figure, electro-mechanical devices are subject to a variety of bias error sources.
Another approach that has been quite common is the "multi-dither" technique, in which multiple images are recorded both before and after one or more adjustments ("dithers") to the various segments locations. The actual changes in the images resulting from the dithers are used to calculate the degree of piston errors present. In order to provide a sufficient number of independent measurements needed to effectively estimate the number of piston degrees of freedom, at least one such adjustment or dither is required for each alignment degree of freedom to be estimated. The implementation of "dithering" can produce complex opto-mechanical arrangements, increase the power and bandwidth required of the segment alignment actuators, and may cause a time averaged degradation of image quality. Such a multi-dither technique is described in R. A. Muller and A. Buffington, J.O.S.A., Vol. 64, #4, 1974, page 1200, and B. L. Ellerbroek, The Optical Science Corporation, Report No. Tr. 645,650, 1985. Because of multi-dither techniques' impact on opto-mechanical complexity, actuator power and bandwidth requirements, as well as sensitivity to noise, the application of these techniques to optical systems with many segments is questionable.
A different approach involves the employment of error sensing devices that are placed directly on the optical surface to be controlled. These include devices such as buried gratings, corner cubes and holographic optical elements. These devices are positioned to reflect light back to a focal plane sensor, with the location of the reflected spot providing information on the relative piston error for the segment from which it was reflected. This approach suffers from limitations similar to the active electro-mechanical measurement technique, discussed above. In particular, surface error sensing devices require complex fabrication techniques, and do not sample the entire surface to be controlled.